1. Field of the Invention
The invention relates to an electric auxiliary heating for a motor vehicle. In particular, the invention relates to an auxiliary heating comprising, in addition to a heating element, also a control unit for controlling the heating power that has to be generated by the heating element.
2. Description of the Related Art
For enhancing the comfort when driving with an open convertible top, heatings have already been used for a fairly long time for preventing draught phenomena in cabriolets. In these heatings, the air sucked in and discharged by a blower is heated making use of the waste heat of the engine or of additional heating modules and is then blown into the passenger compartment of the motor vehicle. In addition, also the vehicle seats can be provided with air discharge openings so as to conduct a current of hot air around the neck area of a vehicle passenger. The heating power of such heatings installed in fittings, seats or other parts of the vehicle is normally manually determined by the user, but the user does not have much influence on the precise temperature and intensity of the air discharged from the heating.
For supplying hot air to the neck area of a vehicle passenger, DE 100 54 009 A1, for example, discloses a wind protection device for an open motor vehicle in which the air current discharged from an air current opening provided in the seat is controlled in dependence upon the vehicle speed. In said DE 100 54 009 A1 the intensity of the air current is increased when the vehicle speed increases, whereas it is decreased when the vehicle speed is reduced. In addition, the heating power is adapted to the changed intensity of the air current so that the temperature of the air discharged from said air current opening is maintained constant. The air current control described in DE 100 54 009 A1 is therefore very complex because the blower as well as the heating power have to be controlled.
Moreover, DE 100 54 009 A1 does not take into account that the heating elements used for heating the air entail inertia effects occurring when the heating power changes. For example, if there is an extreme increase in the air current volume during an acceleration phase of the vehicle, a PTC heating element will, due to its inertia, not be able to provide the heating power required for maintaining the temperature of the discharged air current constant. It follows that insufficiently heated air will be blown onto the vehicle passenger's neck during the acceleration phase, whereby undesired draught phenomena will occur.
Similar problems also arise during heavy deceleration of the vehicle. Due to the inertia of the heating element, the heating power cannot be reduced as fast as the air current volume decreases. Hence, air having an excessively high temperature is blown onto the vehicle passenger's neck during said deceleration.